The proposed research involves the determination of the crystal structures of a series of azapurines, azapyrimidines, and azanucleosides. Some of the molecules to be studied exhibit significant anti-neoplastic activity, while other members of the series do not. The aim of the research is to understand the structural features which contribute to antitumor activity in these and related molecules, so that new drugs which combine many of these features may be designed. In order to achieve this goal, it is necessary to examine the structures (including the intermolecular interactions and hydrogen bonding arrangements) of both active and closely related inactive species, and the aza analogs of nucleic acid constituents provide an ideal system for study. The structures will be solved by single crystal X-ray diffraction methods using standard symbolic addition and tangent refinement procedures followed by full-matrix least-squares refinement.